1. Technical Field
The present disclosure relates generally to electrical and electronic circuits and more specifically to a method of active impedance current sharing.
2. Description of the Related Art
Modern digital electronic circuits rely upon the delivery of significant currents to achieve required power for operation, as the trend has been toward lower voltages. This presents a dilemma in circuit design. Delivering high current loads, e.g., hundreds of amps, to printed circuit boards and integrated circuit devices can raise the cost of circuits. High current buses add significant expense to circuits and devices, but are generally necessary to deliver high current to printed circuit boards.
The information technology equipment being designed today are systems requiring extremely high availability. Powering these systems in an extremely cost-conscious market requires special powering schemes. For redundancy, 1+1 or N+1 configurations are required. A single voltage, or a small number of voltages distributed from the bulk power supply into the chassis simplify the AC/DC converters and allow for bulk power commonality across system platforms, saving development costs. There is an increasing focus on quality and reliability forced by market pressures. Density pressure will continue driving a lower number of output voltages from the bulk supplies. Cost pressures drive more use of Voltage Regulators Down (VRDs) and Voltage Regulator Module (VRM) Common Building Blocks (CBBs).
The need for this equipment to be operational basically 100% of the time mandates that redundant, concurrently maintainable equipment be used. Regardless of form factor and specific electrical requirements, critical electrical features for a power supply in embedded systems are current sharing and the ability to hot-swap. These are the key operational parameters that allow the high-reliability fault tolerance required for today's high uptime systems.
High-availability systems use at least one extra power supply so that failure of one supply does not power down the system. For such an operation, the supplies have to share the load currents. Redundant power conversion designs using multiple converters also require multiple pins for current sharing on +5−, +3.3−, and +12-V outputs. To increase the reliability of the systems and eliminate the possibility of a single point of system failure due to current-sharing connection, some manufacturers employ automatic current sharing that does not require any bus. Passive current sharing is a method of paralleling the outputs of two or more power supplies or DC/DC converters so that they share the load. This is simple, inexpensive and easy to implement. Although passive current sharing provides a highly scalable means of accommodating demands for higher power, one of the converters will always try and output more than half the total current. To implement current sharing between two converters using o-ling diodes, their output voltages ideally need to be adjusted to be perfectly matched under all conditions. However, in the real world, it is nearly always impossible to achieve this degree of trim accuracy.
Droop sharing is also a very simple method to implement. Droop sharing does not require a current share “bus” and works by decreasing output voltage while increasing output current. When the output current of one power supply increases, its output voltage slightly decreases to force the other supplies to take more current. If all supplies are adjusted to the same voltage at a given load and have the same voltage versus current slope, they will share the load with high accuracy. When the load to the power supply rapidly increases, the voltage dip occurs, but with droop it starts at an initially higher voltage level. However, in order for accurate current sharing, the droop method requires a precise voltage setpoint and droop slope. Equal impedance between supplies would be needed as well; realistically this does not happen.
Most embedded systems allot separate pins on power connectors for current sharing. For example, the CompactPCI standard allots three pins for current sharing on +5-, +3.3-, and +12-V outputs. The respective pins are then paralleled on the backplane, which lets the power supplies share information about their currents. What is needed is a highly reliable, accurate method of current sharing. Besides the advantages such as fault tolerance, another benefit to tightly sharing outputs would be for redundant over-current protection (OCP) when two converters are operating in parallel.